Irregular Earth's Inner Core Boundary

The Earth's inner core grows from solidification of the outer core. The solidification process releases latent heat and light elements, providing driving forces for the thermo-compositional convection in the outer core. Because of the presumed extremely small temperature variation in the outer core, the inner core growing process is thought to be geographically homogeneous and geologically slow. The inner core boundary (ICB) has therefore been thought to be flat and simple. In this presentation, we analyze pre-critical compressional waves that are reflected off the ICB (PKiKP), and demonstrate that the Earth's ICB exhibits irregular topography. We collect all available PKiKP-PcP data from earthquakes occurring in Banda Sea recorded at Hi-net stations in Japan during 2004-2010 and apply bandpass filtering and cross-correlation technique to improve the signal-to-noise ratio and ensure the reliability of the picks of PKiKP and PcP phases. PKiKP-PcP differential travel time residuals vary from -2 s to +2.5 s across small regions within the Hi-Net observations. PKiKP/PcP amplitude ratios exhibit large amplitude variation (from 0.02 to 0.29) and a spatially complicated pattern that appears to correlate with the observed differential travel time residuals. Double-event analysis suggests that the PcP travel time residuals are affected little by the seismic structure in the lowermost mantle in the study area. In addition, the PKiKP-PcP differential travel time residuals strongly correlate with PKiKP travel time residuals rather than PcP travel time residuals, further suggesting that the observed PKiKP-PcP differential travel time residuals are mainly caused by the travel time variations of the PKiKP phases. These observations indicate that the Earth's inner core possesses irregular topography. We will also present detailed modeling results of inner core topography based on a hybrid method.